The present invention relates to an aqueous composition for forming zinc phosphate coatings on metal surfaces. When a metal surface is formed, it requires treatment for corrosion resistance and may also require a pre-treatment prior to sealing and/or painting. Zinc phosphate coating solutions are commonly known in the art to pretreat metal surfaces prior to sealing and/or painting. Zinc phosphate coatings isolate corrosion sites. The morphology of the zinc phosphate provides physical anchoring sites for sealing and/or painting solutions which may be applied to the surface in subsequent processing. Zinc phosphate coatings have been successfully used for years to improve the adhesion of paint and other finishes to metal surfaces and to achieve superior corrosion resistance with these finishes.
The drawbacks associated with the application of a zinc phosphate coating solution include the additional process steps which are required in its application. The additional process steps include rinsing and heating the surface after application of the zinc phosphate solution. The rinsing and heating or stoving processes may require a considerable amount of time. The costs associated with these processes include the cost for water rinsing, the disposal of the waste produced after the rinsing process, the equipment and gases and exhaust system used in the heating process, and the processing time used for rinsing and stoving or drying.
The prior art includes solutions and methods for reducing the additional steps associated with the phosphating process and the costs associated with these processes. For example, U.S. Pat. No. 4,659,395 to Sugama et al., provides a process for applying a zinc phosphating solution which does not require subsequent rinsing. However, the prior art of Sugama et al. does require a stoving process to dry the zinc phosphating solution from the surface being treated. The stoving, or other lengthy heating process necessary for drying the surface is time consuming. This is especially undesirable where the metal surface is produced in a continuous process, such as a process whereby a metal sheet is continuously painted at speeds of up to 300 feet per minute, and must be wound around a coil for storage soon after being produced. In this dynamic processing mode, the metal surface must be treated as it is being produced and before it is rolled onto the coil. Therefore, it can be seen that despite advances in the prior art such as those by Sugama et al., a zinc phosphate coating solution which requires neither a rinsing process, nor an extended stoving process for drying, is still lacking in the art.
It is a goal of the present invention to provide a zinc phosphate coating solution which is capable of providing desired adhesion and corrosion resistance qualities by effectively pretreating a metal surface without requiring any additional process operations directed to rinsing or stoving the treated surface.